1. Field of the Invention
The present invention relates to a semiconductor memory device, and particularly to a semiconductor memory device equipped with a ferroelectric capacitor.
2. Description of the Related Art
It has generally been known that a ferroelectric film is subject to degradation of its electric characteristic due to the intrusion and diffusion of a hydrogen gas in various steps subsequent to the formation of a ferroelectric capacitor. Therefore, it has heretofore been practiced to prevent the electric characteristic of the ferroelectric film by covering of the ferroelectric capacitor with a film (hydrogen diffusion preventing film) for preventing the intrusion and diffusion of the hydrogen gas into the ferroelectric film.
FIG. 1 is a diagram showing a structure of a ferroelectric capacitor shown in a non-patent document 1 (Yong Tak Lee, and eight other persons, “Effects of Encapsulating Barrier Layer on Ferroelectric Properties of Ir/IrO2/PZT/Pt/IrO2 Capacitor”, Extended Abstracts of the 1999 International Conference on Solid State Devices and Materials, Tokyo, 1999, (Japan), p. 394-395, 1999). In FIG. 1, in order to prevent degradation of an electric characteristic of a ferroelectric film 43, a structure is adopted wherein a ferroelectric capacitor 40 is covered with a hydrogen diffusion preventing film 35 in which TiO2 and Al2O3 (Alumina) are laminated (see FIG. 1).
As ones available as a material for “a film which substantially does not adsorb hydrogen”, may be mentioned, silver, aluminum, silicon, lead, bismuth, etc. As ones available as a material for “a film which substantially prevents the diffusion of the hydrogen molecules”, may be mentioned, tungsten, conductive oxides, etc. (see a patent document 1 (US Pub. No. US 2005/0051821 A1 (column [0012] to [0013]), which is a counterpart of International Patent Publication No. WO98/31053).
Since, however, there is a need to form a wiring layer 51 connected to an electrode of the ferroelectric capacitor after the deposition of the hydrogen diffusion preventing film 35 in the case of the related art, an additional process step for growing the hydrogen diffusion preventing film is required (see FIG. 1).